Reproductive development and function are complex processes involving both genetically-determined and physiological events. Identification of the critical protein products of genes involved in these processes is necessary to characterize how these processes are regulated. Although important molecular events occur during the early phases of mammalian oogenesis and folliculogenesis, to date, few “candidate” regulatory molecules have been identified and characterized thoroughly. Several studies have suggested that both endocrine factors, such luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the pituitary, as well as paracrine factors secreted from the oocyte influence folliculogenesis. FSH and LH are known to bind to granulosa and thecal cells which in turn are required for oocyte growth and maturation and maintenance of oocyte meiotic competence. Likewise, oocytes may secrete factors which are necessary for normal granulosa cell and thecal cell function. Because oocyte growth is coordinated with the development and growth of the surrounding somatic cells (i.e., granulosa cells initially and thecal cells later), understanding the molecular events at early stages will give important clues about the paracrine factors mediating the reciprocal interactions between oocytes and somatic cells, the development of competence for trophic hormone stimulation, and the process of follicular recruitment.
Disruption of the hypothalamic-pituitary-gonadal reproductive axis by administration of steroids containing synthetic estrogens and progestins has been one of the oldest methods of hormonal contraception. However, the latest report of the Institute of Medicine emphasizes the importance of developing strategies for new contraceptives. According to the report, some of the long-term contraceptive strategies for women include inhibition of ovulation, prevention of fertilization, or blocking of implantation of a fertilized egg into the uterine lining. Furthermore, infertility affects ˜15% of couples, and in ˜40% of the cases, the female is believed to be the sole cause of the infertility. Thus, it is critical to identify novel ovary-specific gene products which could be potential targets for new contraceptive agents.
To identify key proteins in the hypothalamic-pituitary-gonadal axis, we have previously generated several important knockout mouse models, including four which have ovarian defects. Mice deficient in gonadal/pituitary peptide inhibin have secondary infertility due to the onset of ovarian or testicular tumors which appear as early as 4 weeks of age (Matzuk, et al., 1992). Mice deficient in activin receptor type II (ActRII) survive to adulthood but display reproductive defects. Male mice show reduced testes size and demonstrate delayed fertility (Matzuk, et al. 1995). In contrast, female mice have a block in folliculogenesis at the early antral follicle stage leading to infertility. Consistent with the known role of activins in FSH homeostasis, both pituitary and serum FSH levels are dramatically reduced in these ActRII knockout mice. Female mice deficient in FSH, due to a mutation in the FSHÿ gene, are infertile (Kumar et al., 1997). However, these mice have an earlier block in folliculogenesis prior to antral follicle formation. Thus, FSH is not required for formation of a multi-layer pre-antral follicle, but it is required for progression to antral follicle formation. Finally, growth differentiation factor 9 (GDF-9)-deficient mice have been used to determine at which stage in follicular development GDF-9 is required (Dong et al., 1996). Expression of GDF-9 mRNA is limited to the oocyte and is seen at the early one-layer primary follicle stage and persists through ovulation. Absence of GDF-9 results in ovaries that fail to demonstrate any normal follicles beyond the primary follicle stage. Although oocytes surrounded by a single layer of granulosa cells are present and appear normal histologically, no normal two-layered follicles are present. Follicles beyond the one-layer stage are abnormal, contain atypical granulosa cells, and display asymmetric growth of these cells. Furthermore, as determined by light and electron microscopy, a thecal cell layer does not form in these GDF-9-deficient ovaries. Thus, in contrast to kit ligand and other growth factors which are synthesized by the somatic cells and influence oocyte growth, GDF-9 functions in the reciprocal manner as an oocyte-derived growth factor which is required for somatic cell function. The novel ovary-specific gene products presented herein are expected to function in similar ways to regulate oogenesis and/or somatic cell function (e.g., folliculogenesis).